Study of ageing effects in LiF:Mg,Ti by analysis of thermoluminescence glow curves

The effects of ageing on the thermoluminescence of LiF(TLD-100) crystals have been investigated by computerized glow curve analysis. After an annealing of 1 h at 400oC and quick cooling the crystals were aged at 80oC for 0, 6, 20, 60, 200, 800 and 1200 min and readout at 6sup(o)C ssup(-1) or 0.24sup(o)C ssup(-1). The intensities of peaks 2, 3 and 4 decrease with increasing ageing time. Peak 5 initially increases (up to 20%) due to clustering of dipoles and decreases at longer ageing times due to precipitation. Dramatic changes in the values of the activation energies of peaks 2 (from 1.35 to 0.72 eV), 3 (from 1.44 to 1.17 eV) and 4 (from 1.55 to 2.32 eV), have been observed for ageing times longer than 20 min and a readout heating rate of 6sup(o)C ssup(-1). These changes are smoothed out when the readout heating rate is slow. The strong changes of E values indicate a trap modification process which may be caused by precipitation of defect clusters. The relevance for practical dosimetry is discussed. (author)